This invention relates to improvements in a semiconductor luminescent device including two heterojunctions.
There have been previously proposed various attempts to improve the efficiency of energy conversion of semiconductor luminescent devices employing a semiconductor for converting an electrical energy to an optical energy and providing that optical energy as the output. For example, there have been already proposed semiconductor luminescent devices of the type including two hetero-junctions. Some of semiconductor luminescent devices of that type are called TJS laser devices. The TJS laser device comprises a semiconductive body including a first semiconductor layer formed of a first semiconductive material, and a second and a third semiconductor layer formed of a second semiconductive material and having the first semiconductor layer sandwiched therebetween to form hetero-junctions between the first semiconductor layer and the second and third semiconductor layers respectively. Each of the first, second and third semiconductor layers includes a p type region and n type region, and a pair of metallic electrodes are disposed in ohmic contact with the semiconductive body to apply a voltage across each p type region and the associated n type region. The second semiconductive material is selected to be broader in forbidden band-width than the first semiconductive material. Thus the p-n junction of the first semiconductor layer has a diffusion potential less than that of each of the second and third semiconductor layers. As a result, the application of a voltage across the pair of metallic electrodes causes a current to collectively flows through the p-n junction formed between the p and n type regions of the first semiconductor layer resulting in the efficient emission of light from that p-n junction and the vicinity thereof.
Also since semiconductive materials broad in forbidden band-width tend to be small in refractive index, the first semiconductive material has a large refractive index as compared with the second semiconductive material because the latter material has been selected to be broader in forbidden band-width as above described. Therefore light emitted from the p-n junction of the first semiconductor layer and the vicinity thereof is reflected from each of the hetero-junctions until light having a high density energy is radiated through the outer lateral surfaces of the first semiconductor layer. In this way, a high energy light can be effectively emitted from the TJS laser device.
It has been found that an increase in voltage applied across the pair of metallic electrodes on TJS laser devices causes a tendency to increase a leakage current flowing through each of the p-n junctions of the second and third semiconductor layers. This increase in leakage current results in the deterioration of the concentration with which a current flows through the p-n junction of the first semiconductor layer. This leads to the objection that the TJS laser devices decrease in efficiency of light emission.
Accordingly it is an object of the present invention to provide a new and improved semiconductor luminescent device preventing its efficiency of light emission from decreasing due to a flow of leakage current and retaining a high efficiency of light emission even with a high voltage applied thereacross.